One of the most important requirements of a next generation wireless communication system is to support a required high data rate. For this, various techniques such as multiple input multiple output (MIMO), cooperative multiple point transmission (CoMP), relay, etc., have been under research, but the most fundamental and reliable solution is to increase a bandwidth.
However, a frequency resource is in a saturation state at present, and various schemes are partially used in a wide frequency band. For this reason, in order to ensure a broadband bandwidth to satisfy a required higher data rate, a system is designed such that a basic requirement which requires separate bands capable of operating respective independent systems is satisfied, and a carrier aggregation (CA) is introduced. In concept, the CA aggregates a plurality of bands into one system. In this case, a band that can be independently managed is defined as a component carrier (CC).
To support growing transmission capacity, the 3GPP LTE-A or the 802.16m has recently expanded its bandwidth to 20 MHz or higher. In this case, a wideband is supported by aggregating one or more CCs. For example, if one CC corresponds to a bandwidth of 5 MHz, four carriers are aggregated to support a bandwidth of up to 20 MHz. As such, a carrier aggregation system uses a plurality of CCs, and in this sense, can be called a multi-carrier system.
Meanwhile, channel status information (CSI) needs to be fed back for effective communication between a base station and a user equipment. However, a plurality of downlink CCs can be assigned to the user equipment in the carrier aggregation system. In this case, there is a need for a method in which the base station reports to the user equipment a specific downlink CC for which CSI is requested among the plurality of downlink CCs. In addition, how to feed back the CSI by the user equipment is a matter to be considered.